Faults in electric power systems are inevitable. Apart from the damages in the vicinity of the fault, owing to the effects of an electric arc, the fault currents (which are also referred to as “short-circuit currents”) can damage equipment, such as overhead lines, cables, transformers and switchgear. A fault current breaker can limit or switch of a fault current.
A conventional AC fault current circuit breaker includes movable and static contacts. After a switch opening command is given, movable switch contacts move apart, thus creating an electric arc between the movable and static contacts. At a zero crossing point of the current, the arc extinguishes. It will not ignite provided that the insulation distance between the contacts is large enough to rule out. any breakdown of the dielectric between the contacts. If this distance is too small, the arc reignites, i.e., arcing duration in these switches is a function of insulation distance in the arcing area, breaking current value, and tripping time. The arc will continue until the zero crossing point. A majority of known switches are free from any short-circuit current restrictions. However, network-released heat and erosion of contact material during arcing will depend on the current and arcing duration. Arcing duration can be long enough (5 milliseconds as an average). Maximum current value can reach Imax=1.8·√{square root over (2)}·Inominal where Inominal is nominal rated current. This results in an intensive erosion of contact material, and consequently, in a shorter service life of the device and, what is most important, in a smaller number of fault trips. These are the main drawbacks of the device.
Also known in the art is a current limiting switch, also referred to as a “fault current limiter”, which is an electric device that not only commutes rated and overload currents but also breaks fault currents flowing in the protected circuit. Using current limiting switches in existing networks can avoid replacement of the existing network equipment and lines.
A major requirement of current limiting switches is a multiple limitation of an absolute value of fault current in the protected networks. To obtain the operational specifications required, current limiting time must be as short as possible (preferably, tlim<0.8 ms). When power is delivered to the load from the power source (e.g., transformer), the mechanical strength of network wires is tailored both for rated current and fault current. When power consumption increases during the course of natural progress and development thus requiring additional transformers and generators, fault currents increase thus demanding a higher electrodynamic stability of the network and possibly upgrading of lines and equipment.
Various types of fault current limiters, such as passive limiters, solid state limiters and the hybrid fault current limiter are known in the art (see, for example, G. Tang and M. R. Iravani, Paper No. IPST05-158 presented at the International Conference on Power Systems Transients (IPST'05) in Montreal, Canada on Jun. 19-23, 2005; and CIRGE data: Report No. 239 of December 2003 of Working Group A3.10 on “Fault Current Limiters in Electrical Medium and High Voltage Systems”). FIG. 1 shows a circuit diagram of a hybrid fault current limiter including an ultra-fast transfer switch S1 connected in parallel with a load switch that is connected in series with a low-inductive non-linear resistor having a positive temperature coefficient (PCT). Also connected across the transfer switch is a fast-acting disconnector coupled in series with a thyristor bridge that may based, for example, on Gate-Turn-Off (GTO) thyristor or an integrated gate commutated (GCT) thyristor). The three switches are mechanical and during steady state operation of the system, all three switches are closed and the GTO thyristor in the bridge is gated on. When a fault occurs, the ultra-fast mechanical transfer switch opens within several hundred microseconds, and commutates the still rising current into the commutation path, constituted by the disconnector in series with the semiconductor discharge electronic switch. The semiconductor discharge electronic switch provides a time delay for the transfer switch to recover to a certain withstand voltage and is switched off subsequently, forcing the current into the limiting path, constituted by the non-linear resistor. Before this resistor is heated up significantly, thus limiting the current, the disconnector is opened so that the semiconductors are isolated from the continuing rising voltage. Finally, the load switch having an opening time of less than half a cycle interrupts the fault current at its first zero crossing. The time delay between detection of the fault and the limitation of the current can be less than 1 millisecond.
Drawbacks of the hybrid fault current limiter are related to kinematic complexity, high price of the components, relatively low nominal voltage (usually, up to 15 kV), and inoperative nature of the device.
Also known in the art is a synchronous switch (see, for example, Electric control devices, pp. 430, 431, “Vyshaya Shkola” Publishing House, Moscow, 1969), which is an electrical device that commutes rated currents, overload currents, and fault currents flowing in the protected circuit. By using such a device, currents flowing in the protected circuit are interrupted close to zero crossing point (e.g.<1 milliseconds).